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Are aircraft capable of sustained inverted flight at constant altitude?
Inverted flight as a part of manouveres, dogfights, acrobatics is fairly common but my question is:
Are any aircraft capable of sustained inverted flight without losing altitude? e.g. Could you fly 10 mins inverted at the same flight level?
To clarify: I'm not wondering about engines, fuel systems, air-frame strength etc. Fundamentally there's no reason why those systems cannot be engineered to be robust against inverted flight.
What I'm curious about is specifically the aerofoil itself.
i.e. As far as I can see, you'd need to balance the gravity vector with lift.
Ergo, are aircraft wings generally capable of providing enough compensating lift in the "wrong" direction to balance gravity? Are all / most fighters / arerobatic certified craft capable of this?
We'd need an aerofoil that can provide a lift vector pointed in the +z direction normally but changable to a large vector pointing in the -z direction in response to a flight surface deflection?

Just as a clarification, actually being "inverted" doesn't matter, the only thing relevant is if you experience negative G. Staying inverted at a constant altitude would mean sustaining -1 G in the airplanes frame of reference.

I forget who and what plane, but there is a female pilot in the US, who flies 767s from the left hand seat for a living, who has the world record for inverted flight at 600+ miles.  Remember, it's angle of attack that matters.  Most of my flying has been in aircraft with symmetrical wings, albeit helicopters :)

Joann Osterud ?

If you can make it go fast enough, you can fly anything inverted...

Would you believe inverted takeoffs and landings? https://www.youtube.com/watch?v=1ApSDpnA2k0

@curious_cat That's the one, thanks!

@FredLarson Fascinating. I'd never have believed it before I saw this.

I got to see Craig Hosking and Double Take at Eau Claire, WI in 1988. He did two performances during the show. In the first, he took off normally, did an aerobatic performance, and then landed inverted. He taxied in and winched himself out of the plane. Later in the show, he winched himself back into the cockpit, took off inverted, did another aerobatic show, and then landed normally.

Note: you won't find negative "G" maneuvers in a dogfight. Dogfights are about turn radius, turn rates and energy management/conservation. Fighters (nor human body for that matter) are not designed for peak performance under sustained negative g.

Yes they are! Aerobatic aircraft can have symmetrical wing to improve inverted performance. So with these aircraft there is no any problem at all. Other agile aircraft, gliders can fly inverted as well. Of course is far from optimal but possible. Don't forget that lift depends of angle of attack as well. So if you fly inverted angle of attack and drag will be bigger to compensate for a "wrong" wing profile.

Ah yes. I did indeed ignore the angle of attack.

To be fair, gliders can't maintain altitude unless in an updraft. Inverted or otherwise. Gliders are better at flying inverted though, because their engine wont quit. ;)

@falstro their pilots might, though (literally and figuratively)

@curious_cat: Never forget the humble paper airplane. The flat plate airfoil of a paper airplane also generate lift. Airfoil is not about lift. It is about efficiency. If you don't care how much fuel you burn and if you have plenty of excess horsepower feel free to fly with an upside-down airfoil.

I'm far from an expert, but my understanding is that angle of attack isn't just another factor, it's the factor. The shape of an airfoil is just a way of providing a positive angle of attack in a way that reduces parasitic drag. The fact that it's round in the front, curved on the top, pointy on the back basically means that if you drew a line through the "average" of the cross-section, you'd find that it tilts a bit down -- that is, it provides an angle of attack.

Most aircraft use a cambered airfoil. Such an airfoil only gives you a higher stall speed, otherwise it will just cope fine with prolonged inverted flight. The wing's twist will most likely increase the induced drag since the circulation distribution over span is designed for upright flight, and maybe the airfoil will operate outside of its laminar bucket (if its polar has one), so viscous drag will be higher.
If the wing uses a symmetric airfoil and has no washout, like in some aerobatic aircraft, the aircraft will perform identically in both attitudes.
The real limit to prolonged inverted operation is the fuel system and, on some engines, the lubrication. If both are designed for prolonged inverted flight, only the fuel level and the physical condition of the pilot will limit the duration of inverted flight. It is much more comfortable when your body weight is supported by a comfy seat than by two narrow straps running over your shoulders.
The higher stall speed will translate into a higher speed for the minimum power and minimum drag optima, so an aircraft which could keep its altitude in regular flight might have too little power for sustained inverted flight. Also, the trim range of the elevator might be too small to trim the stick forces. In inverted flight the sideslip-induced rolling moment will have the wrong sign, so sideslipping will make you roll into the wind. But if you tolerate all this and engine power is sufficient (which should be true for all fighters since WW II), flying inverted at the same altitude is no particular problem.

Any idea what aircraft Milo Burcham & Joann Osterud used to set their inverted flight records? 4+ hours sounds incredible.

When flying negative G it is much more comfortabe to pull the straps over your legs very tight and leave your shoulder straps quite loose.

@Wirewrap: True, but not for tall pilots. My head would rest on the inside of the canopy if I loosen the shoulder straps.

Areobatics is one of the few walks of life where it pays off to be a short skinny guy.

The problems that you describe are for aircraft which are not designed for inverted flight.  If I'm not mistaken, some aerobatic aircraft have fuel and oil systems designed with pickups on both sides of the tanks just so that they can stay inverted without an issue.  The symmetrical wings also eliminate the problems cause by the wing twist which you describe.

@Lnafziger Yes, correct. I interpreted the question to be about the majority of aircraft and answered it accordingly.

The question though, was "are any aircraft capable of....  Just suggesting that you may want to address that for a more complete answer (which I see that you did).

I realize you explicitly stated:

To clarify: I'm not wondering about engines, fuel systems, air-frame
  strength etc. Fundamentally there's no reason why those systems cannot
  be engineered to be robust against inverted flight.

But I'm going to answer for part of this because it does seem to be the limiting factor of your question.
The main problem with inverted flight is not aerodynamic but propulsive.  Most aircraft fuel tanks require gravity to feed fuel to the engine.  Military and high performance aircraft typically include some mechanisms for maintaining fuel flow during negative G maneuvers like inverted flight.  However, there are usually very strict time restrictions.
For example, the B-1B bomber possesses only a single fuel tank able to operate under negative G maneuvers.  During normal flight fuel is pumped from gravity fed tanks into this special fuel tank.  Then during the negative G maneuvers only fuel from this special tank is available for continued engine operation.
What this means in practice is that aircraft performing any negative G maneuver starts a count-down clock.  When the clock reaches 0 its engine(s) die.
Which is a very bad thing during normally powered flight.
The amount of time any particular aircraft has is dependent upon its particular fuel tank design.

Glad you answered! I didn't mean to make it answer-blocking. :)

Yes.  Modern high performance military and civilian aerobatic aircraft have dedicated inverted fuel and oil systems allowing them to operate in the negative G regimes of their flight envelope without airframe or engine damage.

Something came to mind. Most oil-systems of jet-engines are not capable of sustaining long inverted flight. From the several fixed-wing aircraft I've flown (including jets) the longest limit I've seen (also the most common limit) of sustained inverted flight was 30 seconds. So, the fuel and oil systems would have to be specifically designed for very long periods of inverted flight (or fly electric). Still, I fail to find a practical reason why one would fly inverted for longer periods, even for UCAVs).

When at the top of a loop, you would normally still have positive g. The issue here is to fly inverted sustainably at -1 g. With a symmetric wing (most fighters and some aerobatic planes) this would be easier as it won't try to create lift towards the earth in that instance


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Why does the Falcon 900 have anhedral wings?
Why does it have anhedral wings and not "normal" wings? Many other similar aircraft with low-wing have dihedral wings.

Source: DassaultFalcon.com
Edit: This is about the Falcon 900, which apparently has a low-wing design and it's not used for cargo like the An-225

Possible duplicate of Why does An-225 have anhedral wings though it is a cargo aircraft?

Aside from the size difference, the same effects should apply on the Falcon 900.

@fooot but this has not high wings, so there is a fundamental difference.

@Federico right, that too, but it still has the wing sweep, and the fact that it's only very slight anhedral.

I'm not an aerodynamics expert, but the raised horizontal stabiliser is a clue - T-Tails seem to have pretty neutral dihedral as well.

For the same reason why the Lockheed F-104 Starfighter had such a pronounced anhedral: It is the size and location of the vertical tail.

Lockheed F-104 Starfighter (picture source).
In both cases, the vertical tail is large relative to the wing and fully above the roll axis. In a sideslip it produces a high rolling moment, so any more rolling moment from the wing would reduce the handling qualities of the aircraft.
Especially in case of the F-104, the raised position of the horizontal tail shifts the center of pressure on the vertical tail upwards, exacerbating the offset location of the vertical tail.
The rear-mounted engines on the Falcon 900 move the center of gravity backwards, so the wing is more backwards than usual for aircraft with wing-mounted engines. Therefore, the part of the fuselage ahead of the center of gravity is large, requiring a larger vertical tail to overcome the destabilizing yawing moment of the fuselage.
The same is true for the Tupolev 134 and 154, and - surprise! - they also have no dihedral, even though both are low-wing configurations.

Tupolev 154M of the Polish Air Force (picture source)

I wonder if the DC-9 and B727 would be good comparison points as well....

Picture source
It may seem so when on ground, but in flight the wings flex up. Judging from the photo, the wings seem to have the tiniest bit of dihedral. They also have wing sweep, which provides sideslip-to-roll stability as well.

A swept wing can create a visual illusion re dihedral or anhedral from any angle other than straight-on from the front.  I would be cautious about concluding that that photo shows dihedral.  I would very surprised if the wing flexed enough to remove all the anhedral visible in the photo of the plane on the ground.

It's because anhedral wings remove one of the biggest disadvantages of dihedral wings - maneuverability. Although dihedral wings give roll stability by pushing up on the wing which is lowered while banking, it also means that whenever the pilot is rolling to one particular direction, the wings have a tendency to oppose that by rolling the aircraft to the opposite direction, giving the aircraft reduced maneuverability. But anhedral wings don't come with such a problem.

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Do all GA autopilots use cable "clutches"?
In researching General Aviation autopilots it seems to me that most commonly the way control surfaces are affected is via a servo that attaches (via a clutch) to the control cable. References here and here.
Are there any autopilots that have a servo that directly attaches to, and actuates the control surface(s) as opposed to indirectly via the control cable?

Related: How did old autopilots work?

The pilot is supposed to be able to "overpower" the autopilot causing a disconnect. Once the autopilot disconnects, the servos shouldn't provide any input to the control surfaces. This is a lot easier to do when you run the cables around the servo and use a clutch rather than directly connecting to the control surface.

Sounds like an answer @RonBeyer. Of course, it doesn't answer "all", but it's a good indicator.

From the Approval of Flight Guidance Systems AC-25-1239-1C document:

30. Override of the FGS.

a. General. An override of an engaged FGS function is defined as an action taken by the flightcrew intended to prevent, oppose, or alter an operation being conducted by the FGS function without first disengaging that function.

b. Autopilot override.

     (1) With automatic disengagement.

         (a) Override force. The autopilot should disengage when the flightcrew applies a significant override force to the controls. The applicant should interpret “significant” as a force that is consistent with an intention to overpower the autopilot by either or both pilots. The autopilot should not disengage by minor application of force to the controls, such as a pilot gently bumping the control column while entering or exiting a pilot seat during cruise.

(Page 37)
It goes on to say that 25lbs of force has been determined to be a significant override force level for "other than approach" operations. This may be difficult to measure using a system that just looks at the force on the control surface since normal operations may cause force feedback through the surface. There would have to be a way to measure the pilots force in conjunction with the control surface force and determine if the pilot is trying to override the servo.
GA aircraft are not normally equipped with all the "bells and whistles", autopilots are typically optional equipment. This means that the autopilot system needs to "fit in" to the existing control system architecture. Adding bosses, actuators, connection points, holes, etc to an airframe to accommodate an autopilot is a "significant modification" that may require a new type certificate or an STC.
The easiest way around this is to use servo controls with clutches that move existing control cables. This way the pilots force is directly measurable on the control system. Using magnetic clutches the autopilot can easily disconnect itself, worst case, by popping the autopilot breaker (although this is not an acceptable disconnection procedure).
I can't say that there are no GA aircraft that don't use a different mechanism, but I've never heard of one.

To be able to drive a primary control circuit, the AP uses what is in effect a bi-directional cable winch that allows a small electric motor to apply the normal forces a pilot does.  Pretty much all conventional cable operated flight control systems use that kind of A/P servo.  So you can say that the A/P is driving the control surface directly, just like the pilot, but the A/P servo is a fairly "weak" pilot in that it's maximum force is less than a human pilot can generate, only what is needed for normal control.
The servo's electric motor drives a winch capstan drum through a reduction gearbox, and the winch cables are tied in somehow to the cable circuit itself to be able to pull on either side of the cable circuit, or into a bellcrank in the circuit when the motor is energized in one direction or the other.
There are two clutches between the motor and the capstan drum; an on-off tooth clutch and a slip clutch.  The tooth clutch is two discs with radial gear teeth around the perimeter that intermesh when the discs are brought together.  This clutch is operated by the A/P computer when the A/P is engaged/disengaged by the pilot (or disengaged by the A/P computer for various reasons), and is either fully engaged or not engaged.  They are normally spring loaded disengaged and have to be energized by solenoid to be engaged, so they are normally fail-passive electrically speaking.
But just in case they do jam together for some reason, just upstream or downstream of the tooth clutch is a passive slip clutch that is there in case the tooth clutch can't disengage, so that the control circuit is not completely immobilized by a dead servo that can't disconnect (it also limits the force the servo can apply).
You will be able to move the controls by forcing the clutch to slip, but the effort is very high and it is quite unpleasant to have to control the airplane with controls that feel like they are set in partially cured concrete, but at least you can move them.

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Is a Mode C or S Transponder a type of Mode 3 Transponder?
Mode A transponders are sometimes referred to as Mode 3/A transponders. According to NATO STANAG 4193 (not publicly available, summary from Wikipedia), the "3" in that description refers to the mode in the Identification of Friend or Foe ("IFF") system:

Mode 1 – 2-digit octal "mission code" that identifies the aircraft type or mission.
Mode 2 – 4-digit octal unit code or tail number
Mode 3/A – 4-digit octal identification code for the aircraft, assigned by the air traffic controller (the only mode shared with civilian aircraft)
Mode 4 – 3-pulse reply, delay is based on the encrypted challenge
Mode 5 – Cryptographically secured version of Mode S and ADS-B GPS position.

In flight school I was taught that Mode C was an improvement on Mode A through the addition of data fields that provide aircraft altitude. I extrapolated that to Mode S being an improvement on Mode C through the addition of yet other data fields and making the interrogation targeted at a specific aircraft, and Mode ES/ADS-B being a further improvement through automatic broadcasting.
I have, however, never seen Mode S referred to as Mode 3/S. So, is Mode S a variation within the Mode 3 "protocol" or is it a stand-alone mode outside of the five IFF modes?

Modes 3/A and C are very similar, but the interrogation pulses are timed differently and the responses mean different things.
If you send a mode 3/A pulse, you get a mode 3/A reply (or nothing). The response is a 12-bit number called the squawk code, which is conventionally written as four octal digits, plus an optional ident bit.
If you send a mode C pulse, you get a mode C reply (or nothing). The response is a 12-bit redundant encoding of the pressure altitude between -1200 and +126700 feet, in 100 foot increments.
Note that mode C does not include a squawk code; it is only the altitude. In practice, though, every mode C radar and transponder is also capable of mode 3/A. Likewise, every "mode C" radar always sends both types of pulses and internally combines the two responses, so it looks like a single system. (TCAS only sends mode C pulses since it doesn't care about squawk codes.)
Mode S is almost a completely different system. It includes selective addressing, data frames, message types, and much longer messages (56 bits). It also uses a different, more efficient encoding to allow moving all that extra data without losing efficiency. Of course, for backward compatibility, all mode S transponders can also do mode 3/A and C using the old 12-bit system if needed. Likewise, Mode S radars also send mode 3/A and C pulses, but they're modified very slightly so that mode S transponders will know to ignore them.
1090 Extended Squitter is a relatively minor extension of mode S. A "squitter" means a transponder that sends replies without being interrogated, and "extended" means it added some new message types with 112 bits per message (to hold ADSB data) instead of the previous 56. That's it.

Very nice explanation. So are More C and S still grouped under Mode 3?

@nodapic Not in my view.

In simplistic terms you can view Mode S as an "enhancement" of the Mode 3/C/A.  Mode S changed the way transponders were interrogated by allowing the radar to interrogate one target at a time.  This prevented overlapping returns from multiple closely spaced aircraft that caused positioning errors on the radar operators display.  Mode S is the same format as the Mode C format except that an interrogation code (the Mode S hex code) is transmitted for each interrogation, letting the radar know what aircraft transmitted the return.
Source:  Wikipeida


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Why do aircraft have a crossover airspeed, and why does it increase at higher vertical load factors?
According to the NTSB accident report on the crash of USAir Flight 427, all commercial aircraft have a crossover speed (the speed at which the maximum rolling force from the aircraft’s ailerons and spoilers is just sufficient to counter the rolling force generated by a full rudder hardover; above this speed, a rudder hardover can be weathered with sufficient yoke input, while below this speed, a rudder hardover will cause an immediate loss of control), which increases in step with the aircraft’s vertical load factor (so that the crossover airspeed at, say, 4 Gs will be higher than that at 1 G)1:

Several flight test conditions required the test pilots to maintain control of the airplane and, if possible, a constant (or steady) heading by using the control wheel to oppose full rudder surface deflections. These tests revealed that, in the flaps 1 configuration and at certain airspeeds, the roll authority (using spoilers and ailerons) was not sufficient to completely counter the roll effects of a rudder deflected to its blowdown limit. The airspeed at which the maximum roll control (full roll authority provided by control wheel input) could no longer counter the yaw/roll effects of a rudder deflected to its blowdown limit was referred to by the test group participants as the “crossover airspeed.”
The flight tests revealed that, in the flaps 1 configuration and at an estimated aircraft weight of 110,000 pounds, the 737-300 crossover airspeed was 187 KCAS at one G. At airspeeds above 187 KCAS, the roll induced by a full rudder deflection could be corrected by control wheel input; however, in the same configuration at airspeeds of 187 KCAS and below, the roll induced by a full rudder deflection could not be completely eliminated by full control wheel input in the opposite direction, and the airplane continued to roll into the direction of the rudder deflection. The flight test data also confirmed that an increase in vertical load factor, or angle-of-attack, resulted in an increase in the crossover airspeed.
... The M-CAB flight simulations indicated that, with a rudder deflected to its aerodynamic blowdown limit and in the configuration and conditions of the USAir flight 427 accident airplane, the roll could not be completely eliminated (and control of the airplane could not be regained) by using full control wheel inputs if the airspeed remained below 187 KCAS... To return the airplane to a wings-level attitude, the pilots had to avoid excessive maneuvering that would increase the vertical load factor, or angle-of-attack, and thus increase the crossover airspeed.
... One pilot described how the airplane would initially respond to aileron inputs and begin to roll out of the rudder-induced bank attitude and how, by pulling back on the control column and adding some vertical load factor, the recovery could be stopped and the airplane could hang in a sideslip bank. The test pilot said that he did not apply additional aft column inputs at these moments but that these inputs would have caused the airplane to “roll into the rudder.” The pilot concluded that “you can control roll rate with the control column.” The other Boeing test pilot said that, in referring to the control inputs required to perform a recovery from full rudder input, “there is some technique required between the G [normal load factor] and the roll.”
The flight test pilots affirmed that the Boeing M-CAB and computer simulation models incorporated the tradeoff between normal load factor and roll control but that the tradeoff occurred at a greater load factor in the simulator than in the airplane...
Boeing’s flight test pilots stated that, when they allowed the airspeed to increase to about 220 to 225 KCAS (sacrificing altitude as necessary to maintain airspeed), the airplane recovered easily. The pilots reported that, when they initiated the event at higher airspeeds, the airplane was easier to control and that recovery was accomplished with less roll...  [pages 63-65 of the report/pages 87-89 of the report’s PDF file]
Boeing’s article defined crossover airspeed as the speed below which the rolling moment created by a full lateral control input will not overcome the roll effect from full rudder displacement. The article stated that “while the airspeed at which this occurs is variable, cross-over speeds exist on all commercial airplanes... ”  [page 205/229]
... On the basis of the existing airspeed and the increase in vertical G load, by about 1903:02 the airplane would have been below the airspeed at which the roll controls (aileron and spoilers) could counter the effects of the fully deflected rudder (crossover airspeed). Thus, from that time onward, it would have been impossible for the flight crew to regain roll control without increasing airspeed and/or decreasing the airplane’s vertical G load.  [page 256/280]

Given that an airplane’s ailerons and spoilers generally travel at the same airspeed as the rudder (and, thus, should experience matching increases in control authority with increasing airspeed), how can its lateral control authority surpass that of the rudder past a certain speed?  Even though the aerodynamic forces generated by the ailerons and spoilers - and, thus, the control authority of same - increase as airspeed rises, shouldn’t this same effect cause the rudder’s control authority to increase as well, and stay ahead of the aileron/spoiler control authority?  Why does the control authority of the lateral controls increase with airspeed faster than the rudder’s control authority?  For that matter, is there any particular reason why the rudder’s control authority has to be greater than that of the ailerons and spoilers at low speeds?  The rudder needs to have lots of control authority in order to be able to compensate for an engine failure just above V-1 - but why can’t the lateral controls have even more control authority, in order to be able to compensate for a rudder hardover at low speeds?
And why does an increase in vertical load factor cause the crossover airspeed to increase?  For an airplane to experience a high vertical load factor, it has to be flying at an abnormally large angle of attack - shouldn’t this place the rudder further and further into the wake of the horizontal tail and aft fuselage, “blanking out” the rudder (reducing its control authority) and thus causing a decrease in the crossover airspeed?
..........
1: For the 737, the crossover airspeed also depends on the aircraft’s flap setting, but that’s another question.

how can [an airplane’s ailerons and spoilers] lateral control authority surpass that of the rudder past a certain speed?

This depends on the wing's lift coefficient. At a higher lift coefficient the lower aileron can not add the same amount of lift that it could at a lower lift coefficient. While the raised aileron on the opposite side will still reduce lift locally, the lowered aileron becomes less effective in raising lift as the lift coefficient increases. A higher lift coefficient causes a higher suction peak near the leading edge and puts more stress on the boundary layer, and adding more of the same will become harder as the wing approaches stall conditions.
Another factor is adverse yaw which increases with lift coefficient. This adverse yaw will add to the yawing moment of the hard-over rudder and will increase sideslip, which in turn will produce more rolling moment from dihedral against the aileron effect. As adverse yaw dies down with lower lift coefficient, so does sideslip angle and the ailerons gain control power.
Note that aeroelasticity will lower the effectiveness of all control surfaces as speed increases. Details depend on the torsional stiffness of the fuselage (which will be twisted by the deflected rudder) and of the wing (which will warp against the aileron deflection). Depending where stiffness is higher, the crossover speed and the eventual roll authority at speeds above it might vary.

why does an increase in vertical load factor cause the crossover airspeed to increase?

Because a higher load factor needs a higher lift coefficient when flying at the same speed.

I would agree that the actual "crossover speed" may be where the lowered aileron wing section begins to stall.  Seeing 2 torque curves, the raised ail torque would gradually deteriorate, whereas the lowered ail torque would hold, then drop sharply at stall as AoA increased. Worse yet, it's adverse yaw is on the rudder side too.  Wind tunnel science could help alot.

@RobertDiGiovanni: You are right, adverse yaw is a big factor which I overlooked in my answer. In my view, however, the raised aileron will deliver constant torque (it unloads the wing - should help it to stay linear) while the lowered one will show gradually reducing torque with increasing AoA, details depending on the stalling characteristics of the airfoil.

Sounds like a spoiler on the off side wing would help. I'm actually surprised I've never seen actual aileron torque curves vs AoA published. With a airliner they may have tried more rudder side engine thrust as well, sad it happened.

@PeterKämpf: And it turns out this answers the other question as well!

@RobertDiGiovanni: Unfortunately, jet engines take a while to spool up - almost certainly too slowly to be able to effectively compensate for a rudder hardover.

@Sean  yes, but a full "spool up" would not be necessary on the rudder side. Any extra thrust there would have helped.  It is hindsight, they had only seconds.  One can only hope changes in design, procedures, and training will lessen the probability of it happening again.

Would it be accurate to summarize this answer by saying "it's primarily because the ailerons are more effective when flying at a lower angle of attack"?

@TannerSwett: It's really about lift coefficient. Also, the adverse yaw would matter less if the aircraft has no dihedral effect. So it is not easy to summarize all in one short sentence without leaving important stuff out.

This is an addendum to @Peter's accepted answer, which is the correct answer as far as the B737 is concerned.
Crossover speed is not an industry standard nomenclature. I suspect this term as it's defined here is restricted to the B737 program. Not every aircraft has a "crossover speed" in its current definition.
1. B737 with single rudder PCU
As noted in the NTSB accident report, almost all power actuated Part 25 aircraft have redundant control paths, with two or more independent PCUs. In the case of B737, it has a single PCU (albeit powered by dual redundant hydraulic systems), plus a standby PCU that is normally unpressurized along with the standby hydraulic system. Unless the aircraft detects dual hydraulic failure, reversion to the standby system is a manual process.
In the case of USAir 427 and Eastwind 517, it appears that the root cause of the accidents lies with a jam inside the PCU, which may cause a rudder reversal opposite to the direction of command. Once the rudder reversal is triggered, it feels more like a runaway to the actuator load limit (i.e. the blowdown limit).
Had the rudder been designed with redundant control paths, this rudder runaway would not have occurred due to the other actuators fighting the jammed one.
2. Rudder jam with redundant control paths
However, rudder jam (not runaway) could still occur with redundant active-active control paths. This is something that aircraft manufacturers must consider when certifying for 14 CFR 25.671(c). Manufacturers must show that the aircraft is capable of continued safe flight and landing with the rudder jammed.
But the position at which the rudder is jammed must lie somewhere the pilot (or automatic control) has previously commanded. As you can imagine, commanding rudder to full travel (i.e. the blowdown limit) is exceedingly rare, and having a jam there would be extremely improbable over the lifetime of the aircraft. Other factors also prevent rudder jamming at its blowdown limit; for example, many aircraft have rudder travel limiter or rudder control limiter.
Therefore, we typically do not need to consider rudder jammed full for certification. The exact position for jam consideration has been an active topic and been subject to an ARAC task. Generally, the jam position is maneuver and flight phase dependent.
To summarize, due to the varied nature of rudder jam, there may not be a single crossover speed (in the B737 definition), nor does every aircraft have such a speed.

This seems to be a question of aileron blanking vs rudder.  Higher AoA will blank out upward raised aileron. This is what happens at lower speeds and greater G "vertical loading" with elevator.  Possibly contributing would be that the elevator is also deflecting airflow diagonally across the rudder, increasing its effective length.
Although rudder is blanked at higher AOA, the effect may be greater on the ailerons.
This could be tested in a wind tunnel for possible consideration of re-design.
Larger ailerons or dual rates for slower speeds may provide a solution, as well as evaluation
of the rudder(s).
Post Script:  essentially the aircraft went into a cross controlled elevator aft stall.  Perhaps
newer technology (such as modifying a set of wing spoilers into clamshells) to back up the rudder function would help.

Pretty amazing that one of the comments from the investigation was that "all airliners  have a crossover speed"!


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Why was the tip of the B-52's vertical tail chopped off in 1959?
When Boeing switched over from the B-52F to the B-52G in 1959, they (among other changes) lopped off the top 2.4 meters (8 feet) of the aircraft's vertical tail:
Original-height tail (here on a B-52F)

(Image by the United States Air Force, via David Legrand at Wikimedia Commons, edited by Denniss, Hohum, and Ralf Roletschek at Wikimedia Commons.)
Pruned tail (here on a B-52H)

(Image by the United States Air Force, via Trevor MacInnis at WIkipedia, via Rcbutcher at Wikimedia Commons.)
Why did they chop off the tip of the vertical tail, thereby decreasing the aircraft's directional stability and rudder authority?

You can have too much directional stability, which gives an airplane a strong spiral tendency because the airplane wants to weathervane immediately before dihedral effect can do its thing.  That was probably the case here.

Of because there was a hangar it needs to fit inside of

"decreasing the aircraft's directional stability and rudder authority" Do you see how small of a rudder the B-52 has, especially in the upper part? I'm guessing that this has more to do with the 1964 Savage Mountain Crash and the test where the tail broke off.

Elephant Mountain crash also shows that there were structural issues with the B-52 tail.

@RonBeyer: The teeny tiny rudder makes it even more surprising that they would give up some of what little rudder authority they had to begin with.  Also, if the tail is prone to breaking off, the usual course of action is to make the tail stronger, thus fixing the problem at its source without introducing new ones.

The easiest fix would probably be to make the rudder shorter, to reduce the lever arm and stresses. Redoing all the structure and adding weight is much more complicated. I'd argue that the rudder authority wasn't needed, so they made the tail stronger by making it shorter and fixed the problem without introducing new ones that way.

Boeing B-52G Stratofortress

In the design of the B-52G, considerable attention was paid to reducing the structural weight. Different materials were used in the construction of the airframe, and the wing structure was extensively redesigned. The most visible difference was a vertical tail which was reduced in size. The height was reduced from 48 feet 3 inches to 40 feet 7 inches, and the chord (width) was increased. The new tail was tested on the first B-52A (52-001) and perhaps also on either the XB-52 or YB-52 before being adopted as standard for the B-52G.

This confirms that it was indeed made smaller, but doesn't really address the "why"...

When I read the entire linked article, it seems to imply that many weight saving changes were made, and the smaller tail was just the most visible "weight saving change".

What that article is missing is that the mechanical yaw damper was invented and they were able to use artificial stability to help reduce the needed tail size. The short tail is one of the reasons the re-engine attempt in the 1990s with 4 RB211s was abandoned, as it would make engine out controlability poor.

You might want to add that the 52G was redesigned for low-level bombing and that on several occasions with earlier versions the tail broke off in low-level flight.


text:
Why does an ADIRU 1 failure disable all functions of the A330/340’s GPWS - even those not requiring ADIRU data?
Among the many (many many) aircraft systems degraded or rendered unavailable to the flightcrew of QF72 as a result of a partial intermittent ADIRU 1 (Air Data / Inertial Reference Unit 1) failure, the aircraft’s GPWS was rendered totally inoperative (all emphasis mine):

0440 [. . . . .] NAV GPWS FAULT [. . . . .] The GPWS required ADIRU parameters such as true track angle, computed airspeed, true airspeed and roll attitude, and it was only connected to ADIRU 1. A problem with ADIRU 1 resulted in a loss of the GPWS function. [ATSB Transport Safety Report AO-2008-070, page 48 (page 68 of the PDF file of the report).]
In addition to the potential for a pitch-down command, other known effects associated with the data-spike failure mode on the A330/A340 included:
[...]

unavailability of some other aircraft systems, depending on the ADIRU involved. For example, in cases where ADIRU 1 was affected, the ground proximity warning system (GPWS) was no longer available. [Page 200/220.]

It makes sense that those GPWS functions requiring ADIRU input would be rendered inoperative by a loss of ADR/IR data;1 for instance, excessive-descent-rate protection and terrain-ahead avoidance both require pressure altitude (to calculate the aircraft’s descent rate, in the former’s case, and to determine whether the aircraft has enough altitude to safely clear rising terrain ahead, for the latter), an ADR parameter, while bank-angle protection requires roll attitude, an IR parameter.
However, not all GPWS functions require ADR or IR input; for instance, of the GPWS’s original functions, most (dangerously low height above terrain when not configured for landing; excessive terrain-closure rate; loss of height after takeoff) require only radioaltimeter data2 (plus, for the first, aircraft-configuration data), which is unaffected by even a complete loss of all air and inertial data.
So why does a loss of air-data and inertial inputs disable even the functions of the A330/340’s GPWS not requiring said inputs?

1: Although this raises the obvious question as to why such a safety-critical system as the GPWS was connected to only a single ADIRU, with no ability to switch its inputs to a different source should ADIRU 1 fail.
2: The very factor that makes the radioaltimeter inferior to the baroaltimeter for most purposes (its measuring height above the variably-high ground, rather than altitude above mean sea level) makes it ideally-suited for the GPWS basic functions (as we are interested in not colliding with the ground, rather than with mean sea level; indeed, in some cases, the ground is located below mean sea level).


text:
What are the busiest airways?
Wikipedia lists the busiest city to city routes.  But for instance (nb: this is purely speculation), flights originating in Boston would transit through much of the same airspace to get to LA as a slightly shorter flight originating out of DC.
What are the busiest actual air routes (airways) (irrespective of origin/destination cities)?  Total aircraft count, as well as passenger- and cargo-based metrics appreciated.
Is there a breakdown of aircraft volume transiting between and within the 22 area control centers in the United States?
The best map I've found so far came from this joint publication comparing the US and EU in air traffic management, but this does not show traffic flow between areas:

As originally phrased, I realize such a graph could get very much "into the weeds".  I'm rephrasing the question to start high-level and then work down (if possible).
For example, a flight from Tucson to LA involves a handoff from ZAB to ZLA, such that a numerical representation (aka directed graph) might look like this:
        In
        ZLA ZAB
Out ZLA  X   X
    ZAB  1   X

Adding flights from Tucson to Phoenix (within the same ARTCC) and from LA to Tucson would result in this:
        In
        ZLA ZAB
Out ZLA  X  *1*
    ZAB  1  *1*

Ultimately, this matrix would be 22x22, but since not all ARTCCs can handoff to all others, many cells would be zero.

This is an interesting question - I'm not sure statistics are actually kept on this (at least not by the FAA). If an airway becomes congested it's a simple matter to vector aircraft off the airway (creating new routes on the fly) or assign them a different clearance, so the system is inherently self-balancing...

@voretaq7 Is right, there are too many factors that determine routing between A and B, so there will be no definite answer which is generally true.

The one based on total aircraft count may be available because those metrics are useful for airspace planning. But passenger and cargo based metrics don't make sense for airway capacity.

It won't give you a statistical number, but take a look at FlightRadar24, it can give a nice visual insight into the busiest routes/areas. Some of it may surprise you.

part of my curiosity was wondering if there was a database or tracker for such information.  "too broad" close votes i understand, especially if there's no database tracking this stuff.  i'm questioning the off-topic close votes, however.  one would think this sort of information is relevant to atc.

@DeltaLima for pax vs cargo, i'm curious as to the type of traffic itself, not just the total volume of traffic.  a cargo network often looks vastly different from a passenger network.  memphis or louisville, say, compared to orlando.

I don't agree this question has to be closed. It is clearly defined and within scope. An example answer could be "V123 from waypoint X to waypoint Y". It certainly can be answered, it's just we don't have the data (yet).

This question, while difficult and would be exhausting to answer, is actually very narrow. The answer would be a simple list.

ARTCC data is notoriously hard to find. It is possible, but compiling flight data for this question will take a zealous soul. Perhaps someone could convince flightaware to help.

those sources are acceptable too.  however, it's going to take some special programming on their part and a lot of data retrieval to mark each time an aircraft crosses an area boundary, as well as count aircraft that stay in the same area throughout the flight.

ZOB (cleveland center) always claimed to be the busiest with overflights in the US.  I have no data to back that claim though.

Eurocontrol definitely do keep stats on this, and are running constant tactical projections of airway utilisation across Europe. My guess is that some airways in congested parts of Europe are close to 100% utilisation for a good part of the day.

no idea what are you asking

You can use Flightradar24 which will shows air traffic in real time where you can check which airlines are the busiest. Flightradar24 is using the callsign of the flight and compare it with big databases of airline and airport schedules to find the matching flight number, so not all airlines are submitting their schedules.
You can choose Airline fleet database in Data/History where you can check a comprehensive guide about the fleets of the world's airlines. Once you select an individual airline to view, you'll get its entire fleet. However it won't give you exact number, but list of all airplanes flying per airline (e.g. EasyJet).
To display the number, either you should request that information from the author of the site, or there are few workarounds.

Technical analysis of the busiest airline
When you open the page with specific airline (as suggested above, e.g. EasyJet), you can type the following command in JS console (e.g. in Chrome - More tools and choose JavaScript Console):
$("#listAircrafts li").length

and it'll print you the number of flying planes (sometimes less, because of the pager).
Or use Yahoo Developer Console and type the following query:
select * from html where url="http://www.flightradar24.com/data/airplanes/ryanair-ryr/" and xpath = '//*[@id="listAircrafts"]/li'

And in Tree View, you'll see number of flying planes at the moment (e.g. count: 323) based on Flightradar24 database.
To combine all lists together, you need a script as below.
In example the following *nix command (it'll take few minutes to run):
$ time for uri in `curl -s http://www.flightradar24.com/data/airplanes/ | grep -o 'data/airplanes/.*-[^"]*'`; do curl -s http://www.flightradar24.com/$uri | grep -o '/data/airplanes/.*-[^"]*' | wc -l; echo $uri; done | paste -d" " - - | sort -nr | head -n20
521 data/airplanes/airbus-aib/
484 data/airplanes/china-southern-airlines-csn/
411 data/airplanes/china-eastern-airlines-ces/
404 data/airplanes/lufthansa-dlh/
341 data/airplanes/air-china-cca/
324 data/airplanes/ryanair-ryr/
306 data/airplanes/british-airways-baw/
259 data/airplanes/turkish-airlines-thy/
254 data/airplanes/air-france-afr/
246 data/airplanes/emirates-uae/
214 data/airplanes/easyjet-ezy/
193 data/airplanes/saudi-arabian-airlines-sva/
186 data/airplanes/air-canada-aca/
184 data/airplanes/air-berlin-ber/
179 data/airplanes/netjets-nje/
176 data/airplanes/aeroflot-afl/
171 data/airplanes/qantas-qfa/
169 data/airplanes/tam-linhas-aereas-tam/
163 data/airplanes/cathay-pacific-cpa/
157 data/airplanes/sas-sas/

It will scan over 800 different airlines from the web and it'll print sorted list of the 20 busiest airways according to Flightradar24 database at the time of scanning. Tested on Linux and OSX.

Technical analysis of the busiest route
In order to calculate the busiest city to city routes, it's more complex that above example and you'll have to write similar script (or spreadsheet) of fetching all departures/arrivals of each airport (e.g. LGW) from available Airport database over world airports page, extract data from the tables, consolidate the airport/destination and sum it up in similar way.
Example extraction from table for LGW airport:

Extraction formula which I've used to list all departures from London Gatwick in Google Spreadsheet:
=ImportHtml("http://www.flightstats.com/go/weblet?guid=34b64945a69b9cac:4bf2fb3c:1246346ea7d:4597&weblet=status&action=AirportFlightStatus&airportCode=LGW&airportQueryType=0%22", "table", 0)

So theoretically you can extract all departures of the airport per Sheet (it's like over 3000), group Destination (so you know at least the busiest destination per airport) and sum it up, do some consolidation between Sheets plus some magic, and viola. That's why it's better to write some script:) If I'd have too much time on hand, of course I could do it.
So in other words, the busiest air route would be the airport which will have the highest number of the same Destination city (if I'm correct).
You can always simplify the work by choosing top 20 airports to give you some idea how does it work (instead of checking all of them).

Conclusion
Maybe it won't answer your question directly (which route is the busiest), but it's an idea how to extract the data of the busiest airlines and similar statistics. As it seems it takes a bit knowledge and time how to do it.
If you find any easier way of extracting that data or different database, let me know:)

Summary of the busiest airline
So based on above data (and ignoring Airbus), it's:
18:00 UTC/GMT (Friday)

China Southern Airlines (484)
China Eastern Airlines (411)
Lufthansa (404)
Air China (341)
Ryanair (324)
British Airways (306)
Turkish Airlines (259)
Air France (254)
Emirates (246)
EasyJet (214)

The data is indicative and it depends on the time of day and week, so I'm not warrant of any correctness of it as this is only the example.

The question asked is about busiest airways, e.g. routes of flight, not the most active airlines.

while this might help, it does not answer the question: it provides the airlines with most aircrafts in the air, while the question asks about the busiest airways

doesn't answer the question, but it is a nifty script!

Added section with technical analysis of the busiest route, or rather how to do it, as it's a bit more complex.

@kenorb The tricky part of the question is that it's about particular airways, which are standard routes that aircraft fly while en route. Aircraft flying between many different city pairs will be on the same route at any given time and it's not even guaranteed that all aircraft flying between the same city pairs will take the same route, so simply determining traffic between given pairs of cities doesn't actually answer the question.

The question (as titled) may be impossible to answer, but here goes...
The short answer
This may be the most intuitive answer of all: Look at a visual map of planes currently in the air.
Theory: What does the question mean?
Assuming there exists only one airway between each destination pair, the busiest airway, by definition, would be the one that connects the busiest destination pair (airports/metropolitan areas with the most departures/arivals).
Since in reality there is an arbitrary number of airways between two destinations, the question doesn't really make sense. Do we count two airways the same if they are parallel and 20 miles apart? Why or why not? It is simply an economic question at this point.
The busiest airway could be one in the 'middle of nowhere', but since it is the only one in its area it gets all the traffic.
Practice: What are the busiest airways between two destinations?
Flightaware will show you recently used IFR routes. Here is an example for traffic going from Seattle to Los Angeles. This gives a summary of how many times a particular route was used.
To find the busiest airways between areas of the country, simply count the number of times each airway was used.
Traffic Breakdown by Center

Is there a breakdown of aircraft volume transiting between and within the 22 area control centers in the United States?

You can obtain custom traffic reports through the FAA's Air Traffic Activity System (ATADS), specifically the Center Aircraft Handled page. There are options to break down traffic by a number of parameters.
I would suggest the following settings to get started:
For Output, select:

Standard Report
Show Aircraft Handled (provides totals broken down by operation type)
Show VFR Advisories (for fun)
Show Domestic (provides a breakdown of domestic departures and overflights)
Show Oceanic (provides a breakdown of oceanic departures and overflights)

It sounds like you don't care about the Dates as much; I have selected 2014.

For Facilities, select all of the Centers. You have to individually check each one.

Under Groupings, select Facility only. If you add others there will simply be extra summary rows in the output.

Click run and enjoy the output. Then try different settings if you want.
Then you can branch out into Terminal operations as well.

so the faa does keep records after all.  good find here.  this database is good for recreating a map (similar to what i posted) of the busiest areas, but not so much busiest airways.  if you can find a way to finagle the data so as to produce how many handoffs each area made to neighboring areas, that would answer the question.

all_licenses: Creative Commons - Attribution Share-Alike - https://creativecommons.org/licenses/by-sa/3.0/


text:
What makes an airplane fly?
How does conventional flight work?
Explain what factors are in play in order to get a plane airborne and stay there.

Related meta.SE discussion

These answers are not entirely complete... in the larger sense, what makes an airplane fly is: money!

So I don't agree that this is a duplicate, as the linked question asks for a simplified answer.  That being said, I also don't agree with the way that the answers that are currently here have been done.  An answer is supposed to be "one definitive answer" and not a series of answers because the order of display is not consistent.  Combining the two existing answers together into one CW would be more appropriate.

@Lnafziger is there any way to link to subsections within an answer?

@erich Not that I am aware of, unfortunately.

@erich I believe it has been requested before on Meta.SE and rejected, but I can't find the thread now.

Four Fundamental Forces
Weight
Weight is the force of gravity acting upon an object's mass.  The mass of an airplane is the sum of all parts that make up the airplane, including fuel and lubricating oils, crew/passengers, and cargo.  In a normal flight attitude, this force acts in the down direction, and it will always act towards the earth's center of gravity.
The airplane's center of gravity, or CG, is the location where the aircraft's mass/weight is evenly distributed in all directions.  Optimally, the CG should be as close as possible to the airplane's center of lift.
Lift
In a normal flight attitude, lift is the force opposing weight, acting in the up direction.  It is produced by the interaction of the wings and fuselage with the surrounding air as the aircraft travels through.
There are three scientific principles at work in the production of lift as it applies to aircraft.

Bernoulli's Principle: As air travels faster, its pressure decreases.  A typical airplane wing is designed like a foil, where the upper surface is comparatively rounder than the lower surface.  Air flows faster and at lower pressure over the top of the wing than air flowing underneath it, actually arriving somewhat earlier at the trailing edge than the air traveling underneath the wing.  This pressure difference accounts for most of the lift, but not all.
The Coandă Effect: Airflow is attracted to nearby surfaces.  Air "hugs" the wing as the wing passes through it.  Additionally, since the airflow over the wing is faster than that below the wing, the net result after the wing has passed through is downward air movement.  As the wing angle relative to the initial airflow (the angle of attack) increases, the trailing airflow is increasingly deflected downward.  (However, too much angle of attack will result in disrupted airflow on the leeward side of the wing, causing the wing to stall, resulting in a loss of lift.)
Newton's Third Law of Motion: A force in one direction caused by an object is countered by an equal force upon that object in the opposite direction.  Air deflected downward off the trailing edge of the wing results in a simultaneous upward force on the wing itself.

Drag
Drag is the force of all friction acting upon the airplane, operating in the backward direction.
Drag comes in many varieties.  Skin friction is the resistance caused by moving a solid object (the plane itself) through the air.  Induced drag occurs at the wingtips, where the high pressure air below the wing flows into the low pressure region above the wing, creating inwardly rotating swirls of air (vortices) behind the airplane.  Additional drag is encountered as an airplane's speed approaches the speed of sound and beyond.
Thrust
Thrust is the force causing the aircraft to move forward through the air.  In most airplanes, thrust is produced by a propeller or jet engine.  This is Newton's Third Law in effect as well: as air is pushed backward, the aircraft moves forward.  This forward motion creates a flow of air over the wing, producing lift.
When an airplane's thrust produced equals its drag, the airplane remains in level flight (barring any pilot control inputs).  When thrust exceeds the drag, the increased airflow over the wing creates additional lift.  When thrust is less than drag, the decreased airflow results in less lift being generated.

Not to sure how much of the lift actually cones from bernoulli and how much from the other two but I think it should read "This pressure different accounts for most of the lift, but not all" to make it clearer.

@Maverick283 it's a community wiki answer, please edit as you wish.

@maverick283 these could be opinionated sources, but walter lewin estimates only around 20% of all lift generated is due to bernoulli (at least on a 747).  ga tends be slightly higher, up to around 1/3.  this is why i left it "some".


text:
What is this very isolated, modern aviation/heliport in Northwest Antelope Valley, California?
There's a modern facility at 118 53' 16" 34 9' 28" or about 9 miles north of Neenach, California. It has 5 hangars - 1 very large, 1 medium, 3 small - with other large buildings and a large water tank (a million gallons?). It sits in the foothills of the south side of the Tehachapi Mountains. The only land access is a meager dirt road, almost a 2 track.
Any suggestions as to what it is?

Any suggestions about what?

What exactly are you asking?

Those don't really look like they are, or ever have been runways. You can see a large X painted on both of them, which indicates they aren't runways now.

This isn't and never was an airport. Its the Tejon Ranch facility that Northrup built in the mid 1980's for research on radar cross sections. It closed in 2010 due to interference from nearby solar and wind installations.

It appears to be the Northrop Tejon Ranch RCS facility. It is used for testing of radar cross sections. There’s a page here with a little more info on it.


text:
Do I use true or magnetic north in headwind component calculations?
For the landing performance calculations, I want to calculate the headwind component. I can do this with the runway heading and the reported wind direction and speed. But do I need to make any adjustments for true vs. magnetic north?
Is the reported wind direction towards true or magnetic north? Is the runway heading towards true or magnetic north?

Related: https://aviation.stackexchange.com/questions/60/when-are-winds-given-with-respect-to-true-vs-magnetic-north

See this question for how runways are numbered, and this one for how winds are reported. Wind directions are where the wind is coming from, not towards, by the way.

tl;dr It depends. Runways are magnetic and reported winds can be either true or magnetic. If you are planning you flight and get your winds from a METAR or TAF, then you need to make a correction to the numbers. If you are getting your winds from the ATIS or ASOS, they are magnetic and no correction is needed.

@JScarry I agree with you, except that there are some nuances. For example ASOS and AWOS are true, and sent to NWS as true, but are converted to magnetic for computer generated voice broadcast.

@mongo I’m not sure I follow your comment. If you are listening to the one-minute weather from an ASOS or AWOS then the wind direction you hear is magnetic.

Surface winds, as reported by the tower are magnetic.  Runway headings are magnetic.  However, METARs are reported true, according to Aviation Weather Services and ICAO.  Therefore one has to perform a heading correction in comparing runway headings to METARs.
Wind directions are normally rounded to 10 degrees, as are runway headings.  However, sometimes runway numbering lags isogonic shifts, so care should be used utilizing runway headings.
It is worth noting that the expression, "If it is written it is true, and if it is spoken it is magnetic," may be misleading.  As an example ASOS and AWOS data is recorded as true, and transmitted as true, but is converted to magnetic prior to being broadcast on computer generated voice messages.
Furthermore TWEB are true, as the underlying reports they are generated from are true (winds aloft, TAF, etc.).
To summarize, runways are magnetic, tower wind checks are magnetic, but ASOS, AWOS and METAR are true, unless transcribed to computer generated voice.

@mongo The one exception to the written/spoken rule is PIREPs. The winds reported in them are magnetic.


text:
How should I interpret the Douglas DC-4 approach climb performance chart?
Can someone explain what "C = .04 Vso2" means as the "denotes required rate of climb?". I assumed the point at 6500 ft altitude and 60,000lbs landing weight will mean that the required rate of climb is within the limitations but after calculating this to an approach climb gradient, the answer does not require the limitation of a minimum approach climb gradient of 2.7%.
So the main question is: What does the dotted line mean and what has the C = .04 Vso2 has to do with it?

I interpret the dotted line to indicate the conditions needed to meet the minimum required performance, as specified in CAR1 (the certification standards used when the DC-4 was originally built.). Conditions to the right of the line are "Good", to the left of it are "No Good".  I.E. if you had a gross weight of 60,000 lbs, and were configured as specified, the aircraft would not be able to meet the required performance at any altitude above ~6400 Ft.
I believe the "C=blah etc" statement is referencing the airspeed used to determine these performance results. C= Calibrated Airspeed, VsO² is stall speed in this specific configuration, and I'm guessing that the ".04" means 4% above said stall speed.
I hope this has only confused the question tremendously, and as solely my opinion, is worth precisely what you have paid for it. YMMV. Do not attempt at home. This offer not valid.

Might be that it means rate of climb is 4% of stall speed, not 4% above it.  As an example if that lower point is around 275ft/min, then that would be 4% of 68 knots (which could be reasonable for Vso?)


text:
Why isn't cabin air quality in airliners directly monitored with sensors?
Airliner air conditioning systems refresh the cabin air volume every couple of minutes. A portion of it is filtered and re-circulated into the cabin, however AFAIK there are no direct cabin air quality sensors on board, only pressure and temperature sensors. This question and this question on this site are related.

When researching for an answer to the oxygen question, I came across the surprising research result from this site: carbon dioxide levels in the first column are pretty reasonable, but oxygen content in the second column is around 12%,  a bit over half of the normal percentage.
A report on fumes and smoke events in Australia shows that these are rare, but that often there is not a direct cause found. Would the fumes be detected by human smell only?

Many fumes/smoke notifications to the ATSB are followed up by a maintenance inspection that does not find a potential source or defect causing the fume/smoke, in which case an Service Difficulty Report would not be submitted to CASA.

Airliners do have smoke detectors on board. A Google search reveals that there are cheap key hangers for sale that detect carbon monoxide. Oxygen and carbon dioxide percentages can be easily monitored. My question is: why do aircraft not contain sensors for direct monitoring of the quality of the cabin air that passengers breathe?

At 8000' of altitude the effective oxygen concentration is about 15.4% (this takes into account the reduced volume of air). Also, the sensor used in that study was designed to operate at sea level, the notes say: "The literature includes a correction factor F = 1013hPa (14.692 psi) / actual atmospheric pressure - a factor that may be especially pertinent at high altitudes or in examining aircraft cabin conditions.". The standard deviation for the sensor is also +/- 10-15%, so I'm not sure how much faith I put in those being totally accurate.

@RonBeyer Please check your sources. From this site While the percentage of oxygen in the atmosphere remains fairly constant up to 70,000 feet, the available amount of oxygen to sustain mental and physical alertness decreases above 10,000 feet. The atmosphere is primarily nitrogen (78%) with oxygen comprising 20.9 percent of the atmosphere. There is 21% oxygen up to 70,000 ft, it's just that the body can use less and less of it as pressure decreases. A measurement will show 21% though.

If the percentage of oxygen has decreased, which component has increased? Related: EASA publishes two studies on cabin air quality --> "The results show that the cabin/cockpit air quality is similar or better than what is observed in normal indoor environments"

The key issue for life is the partial pressure of O2, not the actual percentage.

Why is the O2 percentage so low, even on the control test, prior to takeoff?

@TomMcW That's a very good question. The plane is 66% oaded with passengers and air conditioning would be driven by the APU.

There is no FAR or CS demanding that requirement.
I have worked on a system an "aeronose" to detect contaminants , and it worked,we did a trial I installation in an A320 but there was no legal requirement and therefore no airline interest.


text:
How many active Cessna 172 aircraft (or any other make/model) are there in the United States?
I define "active" in accordance with the FAA's definition: "An active aircraft is one that flies at least one hour during the year."
The Bureau of Transportation Statistics (BTS) provides a summary of aircraft registered based on aircraft configuration (fixed wing, rotorcraft, single/multi engine, turbo-prop, jet, etc.) and GAMA has a report on the annual shipments, but not the totals of active aircraft.
The FAA's downloadable aircraft registration database is cryptic to the point where I can't figure out what a specific entry means, and the online search interface returns data for one search at a time.
Is there a data source that provides the count of a given make and model across the active fleet of aircraft in the US?
(PS: If there is some trick to decrypting the FAA registration database that would be be great to know as well!)

How do you define "active"?

Or any other make/model?  Just ask, "How many different registered aircraft have flown in the past 30 days?" or something similar.  As Pondlife pointed out, you did not define what you mean by active.  Maybe what you actually mean is how many aircraft are current with a valid registration certificate.

Added the FAA's definition of "active".

I don't think that information is available, since there's no requirement to report operating hours (or things like annuals) to some central authority.  It's not uncommon to see "ramp queens" and "hangar queens" that obviously haven't been flown in years.

The FAA database
In the zip-file you find a number of text files, and a PDF file called 'ardata.pdf'. This file is the key to the other documents.
The 'master.txt' file contains all aircraft records and 'ACFTREF.txt' contains all aircraft types and their id's in the register.
The third column in 'master.txt' is 'MFR MDL CODE', which corresponds to the first column in 'ACFTREF.txt' called 'CODE'.
By finding the right aircraft codes you can find the number of corresponding aircraft in the 'master.txt' file.
My procedure
I imported both text files in Excel and filtered the aircraft file to contain only the Reims and Cessna 172 variants. This resulted in a list of 54 variants.
Then I filtered the master file using the codes I found, resulting in 20,789 aircraft. This number is the total number of Cessna 172s that have a registration assigned to them in the United States.
However, not all aircraft with an assigned registration are actually active. For example, N-numbers can be assigned to reserve a registration while the aircraft is being transferred/imported or may be cancelled while the N-number still is assigned. This is also included in the 'master.txt' file as the 'STATUS' column.
I decided to only look for valid registrations (indicated by the letter 'V'), which returned 18,914 results. So there are 18,914 civil Cessna 172s with a valid registration in the United States. The actual number of Cessna 172s in the US will be lower, since US-registered aircraft may also be operated in other countries. And some of the aircraft with a valid registration may not be active, but I assume that number is low, so a guess of 18,000-19,000 active C172s will be accurate.

But registered aircraft need not be active per the OP's definition, since registrations are valid for 3 years, and are inexpensive ($5): https://www.faa.gov/licenses_certificates/aircraft_certification/aircraft_registry/reregistration_faq/

I'm accepting Jordy's answer as it provided the key to getting this figured out.
Below is a first draft of the analysis I was trying to complete. There are some inconsistencies with how the FAA's registration database refers to manufacturer and model; the main data modification I had to make was to combine the different variants of a given model into a single label. For example: All the Cessna 182 variants (182N, 182RG, T182, T182T, etc.) are counted under the Cessna 182 label. Additionally, this only shows the aircraft with a valid registration ("V" in Status Code column).
Some additional cleaning remains to make sure the counts are exact, hence this only being a "draft".

@jamesqf: Your comment is well taken. The assumption here is that if you go through the trouble of maintaining your registration, you'd at least fly one hour a year.


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What could cause both engines on a 737-200 to fail at nearly the same time?
On July 2nd a 737-200 departing from Honolulu experienced engine trouble shortly after take-off. The flight ended with the flight crew having to ditch the aircraft and being rescued by the Coast Guard.
Starting at 4:15 in this video the pilot states that the remaining engine is running hot. This is conjecture on my part, but I wonder whether the cause of the engine running hot has something to do with what made the first engine fail.
So my question is: Are there special aspects about the older 737-200 (that may have been fixed in later models of the 737) that could result in both engines failing at around the same time? I have no knowledge about the 737 in general, but an example could be a joint oil reservoir, a pilot action related to the fuel management, or something similar.
Also, related more-so to flight operations: Why would the flight crew choose to fly out over open water instead of returning and landing, or, at least staying closer to the airport?

It doesn't appear a definitive answer or speculation has started yet but the most up to date info would likely by at AV Herald unless of course the link goes dead

Double engine failures that close together are, about 99.99% of the time, fuel related.  That's normally the only common denominator.  The probability of a system related double failure is beyond one in a billion, the cert requirement.

There can be many causes, and we don’t know for sure until the NTSB report. But in general, when fully redundant systems fail at or around the same time, the most likely cause is a common mode failure. Like the fuel supply, or flying through a flock of geese or volcano ash.

Voting to keep open. There is a methodology in system safety engineering, independent of people's opinion.

At the moment you are asking about a specific accident (Rhodes Express 810), which is still under investigation. Until the final accident report is released by the NTSB, this question is off-topic here. If you want to ask about general reasons, please remove the parts about Rhodes Express 810.

I’m voting to close this question because questions about accidents that are still under investigation are explicitly off-topic as per the [help/on-topic].

@JörgWMittag - Please read the question. The first question is about "Aircraft Design" (the last item on the list of approved topcis in the help center) and the second question is about "Flying technique, maneuvers, navigation, procedures, etc." (the first item on the list of approved topics.) The fact that it is related to Rhodes Express 810 is merely interesting.

Judging by the answers there does not appear to be any specific design aspect of the -200 that would make it more susceptible to this type of event. Thank you for all the thorough answers that methodically walked through the common mistakes!

Generally, without knowing the details of this accident, dual engine failures are typically caused by one of the following:

fuel events

exhaustion
starvation (e.g. due to iced fuel filters)
contamination

ingestion event

volcanic ash
ice
birds
water, lots of it.
trees

flight envelope

operating the aircraft outside the flight envelope

maintenance

lack of maintenance (e.g. not refilling oil)
incorrectly executed maintenance (e.g. refilling the wrong type of oil, incorrect re-assembly)

independent failures; single engine failure followed by:

shutdown of the wrong engine (yes, it happened, more than once)
over stressing the remaining engine

Regarding the last point (overstressing the remaining engine):

If one engine fails, the remaining engine has to provide all the power required to fly.
If the aircraft is heavy loaded and flown in demanding atmospheric conditions (hot, humid, high) this may require full power from the remaining engine.
Damage to the aircraft external surfaces caused by the initial engine failure may increase drag and thus require additional power from the remaining engine
Prolonged operation on full power will stress the engine and increases the risk of failure.

Now looking at the specifics of this accident.
The weather at the airport at the time of the incident was not particularly hot (26 Celsius) or humid (±61 % relative humidity). And being at sea level, the airport is not high either. Note that in case of an inversion layer, the temperature may have been higher at altitude.
The planned flight was approximately 100 NM, so the aircraft was unlikely to be heavy.
Although we can safely reject many of the causes listed above, there is no obvious or very likely cause for this dual engine failure. As always, we have to let the investigation team do their work and wait for the final report.

+1 for the good reminder in the last sentence.

In general there's a few reasons for a multiple engine failure:

Running out of fuel: This has happened more than once when the wrong fuel load was put on the airplane, see the Gimli Glider and the Azores Glider, there's also a case where hijackers forced an airplane to run out of fuel and ditch because they wanted to flee to Australia
Fuel Contamination: Dirt, water or other contaminants in fuel loaded onto an airplane can cause engine failures. I don't have any good links but there were relatively recently a couple of Cessna Citation jets that crashed or made emergency landings because of contaminated fuel
FOD (Foreign Object Debris): Clouds of birds or ash can cause multiple engine failures. The Miracle on the Hudson is a famous example
Pilot Error: There's been at least one case where a pilot shut the wrong engine down. One engine fails or has a fire and the pilot shut down the working engine. See the Kegworth Air Disaster and Transasia 235
Maintenance errors: Mistakes made in servicing engines can cause failures, for instance using the wrong oil, the wrong quantity of oil, contaminated oil, installing the wrong parts, etc.
Coincidence: It's statistically possible that both engines just broke at the same time, although it's extremely unlikely

As two engines approach the end of their service life, it actually becomes more likely that both engines fail within a certain timespan when compared to brand new engines. I believe in this case the second engine was overstressed having to keep the plane in flight alone. The company has a certain rep for its maintenance, if true, that'll pop up as factor in report.

I will not speculate as to the cause of the mishap to which you are referring (Transair 810), however, I believe I can provide some clarity regarding the route over water. The Honolulu airport is located relatively close to Waikiki, a neighborhood with a lot of hotels and apartments. Many airports have noise abatement procedures in place to minimize the impact to residential areas nearby. As it turns out, there is a voluntary noise abatement area off the end of the Reef Runway (26L). This may explain the turn offshore after takeoff. It’s also worth pointing out that ditching (or, god forbid, crashing) offshore is a FAR superior option when compared to the same over a (very densely) populated area. The planned route was to fly the PALAY3 departure, then to the LNY VOR, before flying the CAMPS3 arrival. Obviously, the flight never got beyond the PALAY3. The PALAY3 takes the aircraft initially offshore and away from noise-sensitive areas.
A look at the flight path on FlightAware shows the intended route. While it might’ve made more sense to fly a different route, SIDs aren’t customized. They have to serve a variety of routes. The routes work well for ATC flow control and keep aircraft away from risky or noise-sensitive areas and they then disgorge aircraft toward their destinations. Heading offshore isn’t generally dangerous, as twin engine jets are very safe. Hopefully this sheds some light on why the route was picked.

The oil systems on the two engines are separate, for the obvious reason. If there is (uncontaminated) fuel in the tanks, pilots can't starve the engines without deliberately closing the engine start lever (i.e. shutting the motor down themselves) or getting into unusually nose-high attitudes. The latter don't matter when boost pumps are operating- which is basically always, in flight; nose-high attitude can interfere with gravity-feeding fuel from the main tanks. (But that wouldn't, per se, lead to a "running hot" condition -- rather the opposite.)
Ingesting hail or birds can affect both engines, and can cause damage or failure; damage in the right spot could lead to excessive EGT on the way to engine failure.


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Does a circling approach require "one continuous descent to land"?
I recently heard an instructor state that when circling, the pilot "... must execute a single, continuous maneuver once the landing approach is initiated. Don't level off." In his experience this would be a reason to fail a check ride.
When checking the AIM, I found the following:

5-4-20.f: Circling Minimums: Pilots should remain at or above the circling altitude until the aircraft is continuously in a position from which a descent to a landing on the intended runway can be made at a normal rate of descent using normal maneuvers.

Translating the phrase "continuously in a position" to "continuous descent" seems to me a bit of a stretch. I understand that you don't want to descend below your circling MDA altitude for purposes of obstacle clearance. However, once on final, does a correction for a "below glide slope" condition truly justify failing a student? Wouldn't leveling off be considered a "normal maneuver"?
Is there a CFII out there who could shed some light on this? Are there any additional performance/evaluation requirements that are not codified in the AIM?

For reference, this phrase also appears in CFR Ch. 14, 91.175.

I suspect that from a CFIs point of view, having to "level off" (rather than merely adjust your descent to a slower rate) indicates a serious misjudgement of the approach, and that would be a reason to fail.

The wording you quoted from the AIM, "until continuously in a position," does not make sense to me. It is ungrammatical and therefore unintelligible.
The wording at 91.175 is better:

no pilot may operate an aircraft [...] below the authorized MDA or continue an approach below the authorized DA/DH unless -
(1) The aircraft is continuously in a position from which a descent to a landing on the intended runway can be made at a normal rate of descent using normal maneuvers, and for operations conducted under part 121 or part 135 unless that descent rate will allow touchdown to occur within the touchdown zone of the runway of intended landing;

This is clear to me: If, at any point, you find yourself in some position from which a descent to landing cannot be made at a normal rate and using normal maneuvers, you are in violation of the rule (and, presumably, should initiate the missed-approach procedure).
Nothing in the rule implies that you must be in a continuous descent to the runway, that is, that you are forbidden from leveling off at any point.
(Note that I am neither an aviation lawyer nor a CFII.)

I don't see how to even make it a continuous descent in many cases.
Say you are flying ILS runway 36 circle to land runway 18, with minima 700 ft and you break out of clouds at 800 ft. So now you have to stay at the 700 ft altitude flying all around the airport for about 5 track miles until it makes sense to commence further descent shortly before you turn base.
The “continuously in position” means you have to have continuous clear view of the runway and the area where you'll be manoeuvring through base and final, and be certain you can visually complete the landing from where you are.


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How and when is a VFR flight plan closed when flying from the US to Canada?
I recently flew from US to Canada. As required I filed a VFR flight plan online (at 1800wxbrief.com). The flight plan was activated after I took off by contacting Flight Services at 122.2. They confirmed that the flight plan was activated.
When I landed in Canada I called 1800WXBrief to close the flight plan. But they said that the flight was closed by someone a few seconds/minutes after it was opened. As far as they were concerned the flight plan was already closed. After a few minutes the FBO guys notified me that they had received a call from NavCanada about us and upon notification of the safe arrival of the plane, NavCanada had closed the plan.
Does someone know how this process works or is supposed to work? Why do I need to call two different agencies to open/close flight plans?

You should not need to call two different agencies to close your VFR flight plan. Call the nearest ATC unit, FSS (or a FIC) or a CARS is the appropriate action except IFR flights to an aerodrome served by a control tower or FSS where you are not required to file an arrival report unless requested to do so by the appropriate air traffic control unit (CAR 602.77 (3)).
Section 3.12 of the Transport Canada Aeronautical Information Manual (AIM) echoes the regulations from CAR 602.77 (1) and states:

In order to comply with CAR 602.77, an arrival report for a flight plan shall be submitted to an ATC unit, an FSS (or a FIC) or a CARS as soon as practicable after landing but not later than:
(a) the SAR time specified in the flight plan; or
(b) where no SAR time is specified in the flight plan, one hour after the last reported ETA.

This idea of reporting to the closest unit is also supported by ICAO Annex 2 (Rules of the Air), standard 3.3.5.3:

When no air traffic services unit exists at the arrival aerodrome, the arrival report, when required, shall be made as soon as practicable after landing and by the quickest means available to the nearest air traffic services unit.

Perhaps what is missing from the AIM/CARs is the idea that an arrival report should be filed with the closest ATC unit, FSS/FIC, or CARS. This is because the country receiving the flight (Canada in this case) becomes responsible for alerting services at destination (since it's located within a Canadian Flight Information Region). This concept aligns with the standards in ICAO Annex 11, standard 2.10.1:

Flight information centres shall be established to provide flight information service and alerting service within flight information regions, unless the responsibility of providing such services within a flight information region is assigned to an air traffic control unit having adequate facilities for the discharge of such responsibility

I can't explain why 1800WXBrief would have seen the flight plan "closed" after a few minutes but responsibility for it would have indeed been delegated to Canada after departure. This might appear as closed to them in their system.
The call your FBO received from NAV CANADA was likely from FSS or the nearest flight planning office at the Area Control Centre asking whether you had arrived since they had an open flight plan for alerting services. Once the FBO notified them you were safe, they likely closed the flight plan.

Can you comment on this question also? https://aviation.stackexchange.com/questions/52538/what-is-the-process-for-requesting-flight-following-in-canada

I will have a look. Please accept the answer if it solves your question.

I'm not sure if it is the right answer. Maybe it is correct for flights within Canada. The reason I am not sure is because on my flight back from Toronto to KFRG, I opened the flight plan by calling NavCanada and close it in US by calling 1800WXBrief. So I think for cross border flights these two agencies coordinate and transfer the flight plan to each other's system, simultaneously closing the plan in their own system. I have been meaning to call 1800WXBrief for clarification on this. Maybe before my next international flight :)


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What is the process for requesting flight following in Canada?
In US, the process to get flight following is pretty straightforward. You call the approach/center frequency, request a flight following and you get it.
On a recent flight originating from a Canadian airport (Lachute, CSE4), I contacted Montreal approach after takeoff and requested flight following. They asked me if I had filed a flight plan. I said no, and they asked me to speak to Quebec radio so that a flight plan could be filed, Quebec radio will then give me a number and I am supposed to call Montreal approach again with that number. I contacted Quebec radio and the lady asked me a number of flight plan related questions and then asked me to standby. She never came back. Can someone elaborate on this?

What is the general process to get flight following in Canada?
Is a flight plan required to get flight following?
On another flight originating in Canada (CYKZ - KFRG), I was able to get flight following via Ground Control. So I am guessing a flight plan is not always required. What is so special about Montreal? How am I supposed to know which centers insist on a flight plan before they will provide flight following?

The general procedure in Canada is the same as in the USA: to contact the nearest ATC unit and ask for a radar service.
For any VFR flight beyond 25nm from the point of departure it is mandatory in Canada to file either a flight plan or a flight itinerary before departure - (CAR602.73(2)).
The provision of radar service to VFR traffic is typically at the controller's discretion and dependent on workload.
VFR Traffic in the Montreal area is generally expected to follow the suggestion published in the Canada Flight Supplement to contact the Montreal ACC 30 minutes prior to departure to obtain a discreet squawk code. This procedure becomes mandatory if you hope to penetrate the Class C airspace.
Generally it's worth reading the appropriate national publications and/or getting advice from an FBO at your point of departure when flying in a foreign country (Canada).


text:
What are good alternatives to sunglasses for flying?
I'm a student pilot and I won't be able to wear sunglasses for 5 months... Now, how am I going to fly? I know you don't have to wear them but it's easier (brightness etc. etc.). Are there any alternatives you can give for flying? I've never tried a hat, is that a good option for the moment? Maybe it's a silly question but do sunglass contact lenses exist?

Can I ask why you can't wear sun glasses?

Depending on where you would be flying you could try to fly on days with a high overcast. Flying during night and twilight would certainly be an option as well.

I'm having a nose surgery.. BTW: thanks for correcting me Pondlife!

Slap on Sun Visor?

I didn't know that existed! Many thanks for the suggestion. I think that Might help Abelenky if I can't see anything because of the sun!

How about these that clip onto a hat? https://www.amazon.com/Brett-Bros-Baseball-Flip-Sunglasses/dp/B00B466C1A

You might also consider flying in a high-wing plane. I rarely need sunglasses in my Cessna and use them all the time in the Cherokee.

To address the question about contact sunglass lenses...
Contact lenses are available with custom dyes, used for examples by actors to create eye colors not inherent to the actor. Similarly, contacts can be obtained with tint which would attenuate the visible light, in a manner similar to sunglasses.
You should talk with a contact lens dispenser, as they will have access to the offerings of their suppliers.

I've been flying for over twenty years. I've never once worn sunglasses or a hat in the cockpit, with exception of a view limiting device (i.e. hood). I understand why many pilots wear sun glasses and have nothing against it, but for me I feel like it hinders my vision more than helps.
You can fly just fine without sun glasses. One thing that might help, is to make sure you wipe down your windshield before flight to make sure its clean and clear, in order to reduce glare.
A baseball cap might be a good idea if your airplane doesn't have sun visors.
You should be totally fine. Flying without sunglasses is like flying without noise cancelling headsets. Its a creature comfort, but not absolutely necessary for safe operation.

The nice thing about polycarbonite windshields is that they block the bulk of the UV, which is the primary reason protection sought by sunglasses.
The DPE who administered my private checkride told me that he found that wearing sunglasses created a dependency on them, in his opinion.  I changed my ways, and concur with his finding.  My ophthalmologist agrees.

I thought it was just me.

41 years not wearing sunglasses flying.  The eye has served man for al long time in various environments, without sunglasses.

It wasn't clear from your original post exactly why you cannot wear sunglasses.  I wasn't aware of any restrictions (at least in the United States) on student pilots wearing them.
While @Devil07 is correct that sunglasses are not strictly required for safety purposes, they do offer vital protection against damage to your eyes.  To quote the American Optometric Association's article Why you need sunglasses, you need sunglasses for:

UV protection. The sun's UV radiation can cause cataracts, benign growths on the eye's surface, and cancer of the eyelids and skin around the eyes. UV radiation can also cause photokeratitis, sometimes called snow blindness, which is a temporary but painful sunburn of the eye's surface. Wide-brimmed hats and caps can only block about 50 percent of UV radiation from the eyes.
Blue light protection. Long-term exposure to the blue and violet portion of the solar spectrum is a risk factor for macular degeneration, especially for people who are sun-sensitive.
Comfortable vision. The sun's brightness and glare interferes with comfortable vision. Sunlight affects clear vision by causing people to squint and the eyes to water.
Dark adaptation. Spending just two or three hours in bright sunlight can hamper the eyes' ability to adapt quickly to nighttime or indoor light levels. This can make driving at night more hazardous.

I should note that I actually don't wear sunglasses...I wear prescription glasses with transition lenses that provide some of the protections mentioned above.
You also asked about contact lenses.  The AOA also has an article on contact lenses that provide UV protection.

Any poly-carbonate lens eyeglasses will provide adequate UV protection. Cataracts when the lens of the eye, which is internal, not the surface, clouds or becomes opaque.

do you have any published literature on the dark adaptation times being adversely impacted by two or three hours of bright sunlight?  There are several mechanisms that they eye has for light dark adaptation, and there is a plethora of peer reviewed articles which document the dark adaptation times, and generally, are short. In fact the eye has a dynamic range of 100,000,000:1, in part due to the transition from rods to cones, pupillary response, photopigments and also the receptors themselves. However, normal adaptation to full night vision occurs in 30 to 40 minutes.

@mongo I do not.  Please note I've quoted an article on the AOA website and am not asserting the items they've iterated myself necessarily.

For a better understanding of light adaptation, I would suggest looking at resources which don't have an interest in selling sunglasses.  A colleague has a nice set of lecture notes http://www.cns.nyu.edu/~david/courses/perception/lecturenotes/light-adapt/light-adapt.html which you might find interesting and informative.  I do not believe that the quoted statements are accurate, given my experience as an imaging scientist, and my knowledge of the human visual system.  I am curious what coating your own glasses have which protects light/dark adaptation.

Actually, I admittedly misspoke by saying the coating on my glasses provide all of the protections mentioned above.  I will correct it.  My glasses have transition lenses that do provide UV protection, which I still believe is critical.  If you disagree, I am open minded and willing to learn from someone who knows better about this specific subject than I do.

My first comment agrees that poly-carbonate lens eyeglasses provide UV protection.  Coatings are not needed as the base material absorbs the UV energy. Even common safety glasses may provide such protection. Historically contacts have not, but new technologies are emerging.  The windshield of the aircraft may, depending upon the material, also provide UV protection.